A known method and apparatus for communicating with an underwater vehicle is described in U.S. Pat. No. 5,119,341. A plurality of buoys determine their positions based on Global Positioning System (GPS) navigation satellites and emit acoustic underwater data messages which contains this position. An underwater vehicle receives the messages and determines its position therefrom. Spread spectrum encoding is used to allow a single beacon carrier frequency for all buoys. Alternatively separate and locally-unique beacon carrier frequencies can be assigned to each buoy.